An element is called an organic electroluminescence element (hereinafter referred to as an organic EL element) which includes a thin film of an organic material and two electrodes sandwiching the thin film and emits light in response to application of voltage (electroluminescence). In the 1960s, an organic EL element made of a small-molecular organic material was found (see Non-patent Reference 1), and in the 1980s, an element structure with practical properties and a practical process of manufacturing such an element was developed (see Non-patent Reference 2). The organic EL element made of a small-molecular material may be formed as a thin organic film using a vacuum deposition method and manufactured in a condition with less incorporation of foreign matters or dust in a vacuum process. This provides the organic EL element made of a small-molecular material with characteristics of having a long life and less pixel defects. In the early half of the 1990s, an organic EL element made of a polymeric material is reported (see Non-patent Reference 3). The organic EL element made of a polymeric material can be obtained as an organic thin film by applying solution or dispersion, which is obtained by dissolving a polymeric material in solvent, using a wet method. This provides the organic EL element made of a polymeric material with a characteristic that it may be manufactured through a simple process with less material loss under atmospheric pressure. Both of these organic EL elements are characterized by bright light emission, small dependency on viewing angles, easy increase in panel dimensions and decrease in array size, and developed as light sources for displays or lighting in these years.
FIG. 6 is a cross-sectional view of a structure of a conventional organic EL element described in the Non-patent Reference 2. An organic EL element 600 shown in FIG. 6 includes a transparent substrate 601, a transparent bottom electrode 602, an organic layer 603, and a non-transparent top electrode 604. In this structure, the transparent bottom electrode 602 is stacked on the transparent substrate 601, and light emitted from the organic layer 603 is taken out from the substrate side. The non-transparent top electrode 604, which may be a metal electrode, reflects light emitted from the organic layer 603. An organic EL element which has a structure similar to the structure of the organic EL element 600 is hereinafter referred to as a bottom-emission organic EL element.
FIG. 7 is a cross-sectional view of a structure of a conventional organic EL element shown in the Non-patent Reference 1 and others. In this structure, light emitted from the organic layer is taken out from the top electrode side. An organic EL element shown in FIG. 7 includes a non-transparent substrate 701, a non-transparent bottom electrode 702, an organic layer 703, and a transparent top electrode 704. In this structure, the non-transparent bottom electrode 702 is stacked on the non-transparent substrate 701, and light emitted from the organic layer 703 is taken out from the transparent top electrode 704. An organic EL element which has a structure similar to the structure of the organic EL element 700 is hereinafter referred to as a top-emission organic EL element.
Considering applicability to an active-matrix organic EL display which includes an organic EL element and a thin film transistor (hereinafter referred to as a TFT) which drives the organic EL element, a top-emission organic EL element is superior to a bottom-emission organic EL element. This is because the area of a light emission portion in a pixel area of an organic EL element is limited to an area other than the area of a non-transparent TFT and electric wiring on a substrate of a bottom-emission organic EL element in the case of which emitted light is taken out from the substrate side thereof. At the time, design flexibility is limited because the area of the TFT and electric wires within the pixel is required to be as small as possible in order to secure the area of the organic EL with priority.
In the case of a top-emission organic EL element, in contrast, emitted light is taken out from the side opposite to the substrate side thereof, so that the organic EL element can be formed on a TFT layer over the substrate side and the area of the TFT layer may be made as large as a pixel area. With this, the channel width of the TFT is increased, so that current provided for the organic EL element increases. Alternatively, the TFT is increased in number to form a current compensation circuit, so that luminance distribution is made even across the display. In addition, as the area of the organic EL element is increased in proportion to the pixel area, load for light emission per unit element decreases and the life of the display is improved.
Especially for a top-emission organic EL element, which has a great advantage in application to the display, an electrode made of a metal oxide which transmits visible light and has good conductivity, such as indium tin oxide (hereinafter referred to as ITO), is used as the transparent top electrode 704. Because it is difficult to form such a metal oxide into a thin film with good transparency and conductivity by resistance-heating vapor deposition, the film is formed using a sputtering method or plasma.
In a general structure of an organic EL element, a bottom electrode is an anode and a top electrode is a cathode. Especially for an organic EL element made of a polymeric organic material, a polymeric layer is formed using a wet method such as a spin coating method or an inkjet method. Alkali metals, alkali earth metals, and salts thereof used as a cathode with a function to provide electrons are likely to be unstable when reacting with water or oxygen. Accordingly, when a bottom electrode is a cathode, alkali metals, alkali earth metals, or salts thereof included in the bottom electrode reacts with an organic layer, which is a solution layer early in its formation, and causes mutual elution or mutual diffusion at a laminate interface therebetween; resulting in difficulty in control of the laminate interface. In this regard, a structure in which a top electrode is a cathode is employed.
However, a top-emission organic EL element with a top electrode of a transparent electrode has a problem.
The problem is that the sputtering method or the method using plasma for forming an ITO film, which is the transparent top electrode 704, causes great damage to the organic layer underlying the ITO film. This damage causes negative effects to the element, such as instability of driving, deterioration in luminous efficiency, increase in driving voltage, and shortening of the life.
In addition, the transparent electrode made of a metal-oxide such as ITO has an excellent hole injection property owing to its high work function but is not easy to inject electrons. Accordingly, in order to inject electrons through such a transparent electrode of a metal-oxide such as ITO, which is not easy to inject electrons by nature, an electron injection layer which promotes electron injection is required to be provided between the electrode and a luminescent layer.
It is known that alkali metals and alkali earth metals effectively function as the luminescent layer. The alkali metals include lithium, sodium, calcium, rubidium, cesium, francium, and the alkali earth metals include magnesium, calcium, strontium, barium, radium. However, in the case, for example, where an electron injection layer made of barium and an ITO layer contacts each other to form a laminate structure, the barium contacting the ITO layer is oxidized because the ITO is an oxidizer. Thus electron injection performance is significantly deteriorated and the device does not function.
In order to solve this problem, it is effective to provide some buffer layer (hereinafter referred to as a cathode buffer layer) between an electron injection metal and an electrode of a metal oxide typified by ITO.
The following are performance requirements for the cathode buffer layer:    (1) Protect an electron injection metal from oxidization by the transparent cathode (for the purpose of improvement of stability of the element).    (2) Have high transparency (for the purpose of efficient extraction of emitted light generated in the element to the outside of the element).    (3) Protect the electron injection layer and the luminescent layer from damage in processing for forming a film of the transparent cathode (for the purpose of stable drive, high luminous efficiency, and a long life of the element).    (4) Provide excellent transport and injection of electrons from the transparent cathode to the electron injection layer (for the purpose of drive at a low voltage).
For example, a cathode buffer layer for a conventional top-emission organic EL element is disclosed in Patent Reference 2. According to the Patent Reference 2, an organic material which is a mixture of metals with low work functions and has an electron-transport property is used as a cathode buffer layer under a transparent electrode. Mixing metals with low work functions reduces (n-dopes) the electron-transporting organic material which causes radical anion status, thus generating free electrons in a film. Thereby electron injection from the transparent cathode is promoted and conductivity of the cathode buffer layer is increased. This means that the aforementioned performance requirement (4) for the cathode buffer layer is satisfied.
Furthermore, the cathode buffer layer has good transparency. This is owing to the fact that many electron-transporting organic materials have good transparency and the transparency is enhanced because the mixed metals are oxidized to be cationic. This means that the aforementioned performance requirement (2) for the cathode buffer layer is satisfied.
Furthermore, as the film can be thickened without increase in driving voltage or significant decrease in luminous efficiency because of high conductivity and good transparency, the electron injection layer and the luminescent layer are protected from damage due to processing in forming the transparent cathode film. This means that the aforementioned performance requirement (3) for the cathode buffer layer is satisfied.
Patent Reference 1: Japanese Unexamined Patent Application Publication Number 10-162959
Patent Reference 2: Japanese Unexamined Patent Application Publication Number 2004-127740
Non-patent Reference 1: M. Pope et al., Journal of Chemical Physics No. 38, 1963, pp. 2042-2043
Non-patent Reference 2: C. W. Tang and S. A. Van Slyke, Applied Physics Letters, No. 51, 1987, pp. 913-915
Non-patent Reference 3: J. H. Burroughes et al., Nature, 347, 1990, pp. 539-541